Comparisons of Neuronal Response Reliability Between V1 and V2 of Infant Monkeys

Abstract

Purpose: : A recent study suggested that both the firing rate and response reliability of individual cortical neurons influence the fidelity and efficiency of information transmitted by individual V1 neurons, and in infant monkeys, the poor firing rates are compensated by higher response reliability (Rust et al, 2002). In this study, we determined whether similar relationships between firing rates and response reliability exist in V2 and if so, whether response reliability is greater in V2 than in V1 in these infant monkeys. Methods: : Microelectrode recording experiments were conducted in V1 and V2 of 2–, 4–, and 8–week–old monkeys to. Receptive fields of all units were located within 4.0 degrees of the center of the fovea. Drifting sine–wave gratings, optimized for each unit with respect to size, spatial frequency, and drift direction, were presented as visual stimuli on a uniform background with mean luminance of 50cd/m2. Responses were measured for gratings of six different contrasts ranging from 0 to 0.5. At each contrast level, responses were obtained in each trial during a period of 640msec for 30 trials. Mutual information was calculated for all contrast pairs and was plotted against the difference in firing rates of each pair. Information density was derived from the fitted curve. Spike count variance to mean ratio during the 640 msec trial period for each contrast was also computed. Results: : V1 and V2 neurons in the first 8 weeks of age showed lower firing rate, but significantly higher information densities and lower variance–to–mean–ratios than those in adults. While there were no significant differences among V1 neurons among infant age groups, V2 neurons at 2 weeks of age showed significantly lower firing rates, higher information densities and lower variance–to–mean–ratios than those in 4– and 8–week–old infants. Conclusions: : As in V1, the lower firing rates of V2 neurons were compensated by their lower response variability, but this reciprocal relationship was far more exaggerated in V2 at two weeks of age than in V1. These results provide additional evidence to support the hypothesis that the functional maturation of cortical neurons proceeds in a hierarchical order